1. Field of the Invention
This invention relates to an extremely compact low voltage apparatus for generating high current subnanosecond duration electron pulses. More particularly, a short compact electron gun is described which has: (1) an electron emission area, (2) an electron beam gate which is either open or closed, (3) a beam deflection area which is maintained essentially free of space charge, (4) a small electron sweep aperture positioned at a relatively long distance away from the deflection area and located with the opening on an axis, and (5) a pulse detection circuit. The aforementioned structure is surrounded by a direct current magnetic collimating coil, which coil collimates the electron beam and thus increases its intensity and energy content.
Our invention employs a conventional low voltage pulse generator to deliver an electron gating pulse to the electron beam gate. That pulse has a magnitude which exists for a finite short time duration. A burst of electrons is gated "on" only during that finite duration. The energized magnetic coil accomplishes its collimation function and a tightly bunched electron beam of finite duration is thus formed. The electron beam is "off" in the absence of the electron beam gating pulse.
In our invention a bias condition is established such that when gated "on," the electron beam is normally held in an off-axis position. This off-axis bias condition results from a deflection potential which establishes an electric field between a pair of deflection plates in the deflection area. The direction and polarity of that field holds the beam in the off-axis position so that the beam cannot reach the beam detection circuit. A second pulse, time-delayed from the first, forms a beam deflection electric field in a direction opposite to the first field between the pair of deflection plates. That second field has sufficient polarity and strength to sweep the electron beam smoothly and quickly across the sweep aperture. This sweep, as is described in more detail in the specification, is termed a symmetrical sweep.
In accordance with our invention and the described operation a very high peak instantaneous current electron beam pulse is generated. In one preferred embodiment of our invention, that pulse is approximately 70 microamperes with an extremely short duration, or width, of approximately 0.35 nanoseconds.
2. Background Discussion
Electron pulses having high peak instantaneous current and subnanosecond duration are of great value in several diverse areas of the art. For example, such pulses are essential for measuring certain excited gas-phase species of interest. Some excited species, to give a typical example, may have lifetimes in the order of a few nanoseconds. Measurement of these lifetimes, within satisfactory error tolerance, requires that the electron beam pulses used to change the atomic states of the excited species under study must be as short as approximately 10% of the lifetime to be measured. If the state-changing electron pulse is too long, measurements of the decay rate of the atomic states will include a severe contribution from the shape of the electron pulse and consequently result in erroneous lifetime measurements.
Outer-space and similar environments pose severe weight and power limitations on the design of electron guns. Of course, pulsed beam electron guns are known in the prior art. Generally, however, such guns require kilovolt operational voltages and are available in lengths of several tens of centimeters. Obviously such guns, from a practical standpoint, have limited application when a compact, low power, relatively inexpensive electron gun is required.
Other applications in the art which require high amperage, short duration electron pulses include digital high definition television imaging systems which are under current investigation by the television industry. In many industrial applications such as visual displays, plasma switches, microcircuit etching, etc., a low cost electron gun is highly advantageous.
Conventional guns such as are described, for example, in U.S. Pat. No. 2,462,860 to D. D. Grieg exemplify the large-sized high-powered guns of the prior art. Implicit in such prior art is electrostatic focusing of the electron beam. Such electrostatic focusing inherently requires very large and lengthy deflection plates that represent a high capacitance in the system. Such high capacitance detracts from the ability to generate short duration pulses.
The prior art very often employs power supply voltages in the order of several kilovolts. The resulting structures of the prior art generally yield electron pulses with peak instantaneous currents of only a few microamperes while the pulse widths are normally in the order of several nanoseconds duration.
Attention is called to a U.S. Pat. No. 3,402,357 issued to J. Haimson, et al. which is directed to an apparatus for producing electron beam pulses having nanosecond or fractional durations thereof. See, for example, Col. 2, lines 40 through 48. Such structures are extremely complex and rely upon costly and cumbersome beam focusing lenses and a precisely controlled sequence of thyratron discharges in an effort to reduce the duration time of the electron beam pulse being generated. The complexity of this, and other similar prior art devices, severely limit their wide-spread applicability when a compact, low voltage electron gun is required.